Abstract
The main ideas for the formation of large interstellar cloud complexes and the role of massive stars in the sta formation cycle are reviewed. Massive cloud complexes can be formed via large scale instabilities or by the action of strong perturbations evolving in a sheared disk. The energy injected by new stars can destroy their parental clouds and, hence, the star forming activity in these clouds is self-limited. The most efficient cloud destruction mechanism is due to photoionization from massive stars, which limits the population of massive stars. The maximum number of OB stars that can form within a dense molecular cloud fragment is of order 10 per 10 M0. Assuming a filling factor of 0.1 for these dense fragments, the resulting star forming efficiency within cloud complexes is roughly 5%. Once the parent cloud is dispersed, the combined action of stellar winds and supernova explosions from OB associations generate large expanding bubbles. These multisupernova remnants are responsible for the bulk properties of the general interstellar gas, and can control the overall star forming activity in isolated galaxies. Key words: ISM - CLOUDS ISM - SUPERNOVA REMNANTS - STARS - FORMATION
